1. Field of the Invention
The present invention relates to a transmitting apparatus that externally transmits a radio signal which contains at least a main information portion, a receiving apparatus that receives the radio signal, and a body-insertable apparatus system.
2. Description of the Related Art
In recent years, a swallowable capsule endoscope is proposed in the field of endoscope. The capsule endoscope is equipped with an imaging function and a radio communication function. The capsule endoscope is swallowed by a subject from a mouth for an observation (examination). After being swallowed, the capsule endoscope moves through inside body cavities, for example, internal organs such as a stomach and a small intestine, following a peristaltic motion inside the subject, and sequentially picks up images inside the body cavities until being naturally discharged.
While moving through the body cavities, the capsule endoscope sequentially transmits data of the picked-up images of the inside to outside by radio communication. The transmitted data is stored in an external memory. After swallowing the capsule endoscope, the subject carries a receiver which has radio communication function and memory function until the capsule endoscope is discharged, and can move freely. After the capsule endoscope is discharged, a doctor or a nurse can retrieve the image data stored in the memory and watch images of the internal organs on a display monitor to make diagnosis (see Japanese Patent Laying-Open No. 2001-231186).
In the conventional capsule endoscope system, the image data picked up by the capsule endoscope is transmitted by radio in a similar data structure to a structure adopted in an NTSC image transmission, for example. Specifically, the conventional capsule endoscope system configures image data which corresponds to one image frame by arranging a horizontal blanking period between data corresponding to a predetermined scanning line and data corresponding to a scanning line adjacent to the predetermined scanning line. The horizontal blanking period is originally provided as a gap which prevents a retrace line from appearing in an image displayed on a cathode ray tube display. The radio signal transmitted from the capsule endoscope has a gap portion corresponding to the horizontal blanking period. The gap portion is made up of a direct-current component which corresponds to logical HIGH or logical LOW in a digital signal.
The conventional capsule endoscope system, however, has a following problem which is attributable to the presence of the gap portion in the radio signal transmitted from the capsule endoscope. The problem caused by the gap portion in the radio signal will be described below in detail.
Firstly, when the conventional capsule endoscope system, which employs the radio signals with the gap portions, uses an AC coupling in a signal processing circuit or the like included in a receiver, instant voltage changes may create problems. A main information portion includes a signal component, which has predetermined amplitude and corresponds to the image data, and an average level of the main information portions corresponds to the signal components. On the other hand, the gap portion has a constant voltage corresponding to logical HIGH or logical LOW. Therefore, the average level of the main information portions generally differs from the average level of the gap portions (i.e., the voltage level corresponding to logical HIGH or logical LOW) by a predetermined amount. The receiver which is to receive the radio signals usually includes an AC coupling circuit including a capacitor and the like in order to reduce an offset that corresponds to the voltage difference.
When the AC coupling circuit as described above is employed in the receiver of the capsule endoscope system, instant voltage changes such as sags can be a problem. When the radio signals are transmitted from the capsule endoscope, voltage changes frequently occur during the reception of image data corresponding to one image frame, since the image data is configured so that the main information portion and the gap portion appear alternately. Then, the AC coupling circuit may have difficulty in following the signal transition, for example.
Further, in the conventional capsule endoscope system, a noise component is difficult to remove from a radio signal received by the receiver at the receiver side. As described above, the gap portion includes a direct-current component of a predetermined voltage level. Therefore, the radio signals transmitted from the capsule endoscope include low frequency components corresponding to the gap portions by a predetermined rate. The horizontal blanking period generally lasts for a few hundred microseconds (μsec) and direct-current signals of a predetermined intensity are output during the horizontal blanking period. Therefore, the radio signals inevitably include low frequency components corresponding to such a time length.
On the other hand, the receiver is usually provided with a frequency filter which serves to extract only the radio signals transmitted from the capsule endoscope by removing the noise components from the received radio signals. The frequency filter has a function of extracting a band of frequency components corresponding to the frequency of the radio signals transmitted from the capsule endoscope. When the radio signals include low frequency components in the gap portions, the frequency filter is required to accommodate a wide band so as to pass the low frequency components of the gap portions. Then, the amount of noise components that can be removed by the frequency filter decreases. Therefore, to make the frequency filter function effectively, the radio signals preferably do not include low frequency components.